The present invention relates to a fuel cell, and more particularly to a fuel cell having at least a gas diffusion electrode having an electrode catalyst prepared by uniformly distributing and depositing noble metal particles on catalyst carriers. The present invention is suitable for a phosphoric acid fuel cell or a methanol fuel cell.
Sites of reaction among an aqueous solution containing ions that take part in the reaction, an electron conductor that can pass electrons therethrough, and a reacting gas such as hydrogen gas or oxygen gas are given at both anode and cathode in the case of the phosphoric acid fuel cell, and at the cathode in the case of the methanol fuel cell, where the electrodes work as three-phase boundary electrodes of liquid, solid and gas, the so-called gas diffusion electrodes. The gas diffusion electrode is prepared, for example, by coating carbon paper of carbon fibers as a substrate with carbon powder supporting electrode catalyst particles of noble metal such as platinum by means of a binder such as polytetrafluoroethylene i.e. Teflon.
The electrode thus prepared has a good gas permeability, and can retain the aqueous solution containing ions within the electrodes thanks to the surface tension due to water repellency of polytetrafluoroethylene and hydrophilic properties of carbon without permeation of the aqueous solution to the other side of the electrode.
The gas diffusion electrode has many pores, and forms three-phase boundries where the three phases of liquid, solid and gas are in contact with one another, and the reacting hydrogen or oxygen gas can diffuse therein to undergo reaction at the boundaries.
To increase a current density, it is necessary to form more site of such reactions (active points), thereby increasing the activity of electrode catalyst.
Generally, the activity of noble metal electrode catalyst can be increased by making noble metal particles finer and uniformly distributing the finer particles onto carriers.
Many procedures for preparing noble metal electrode catalyst have been so far known, and the ordinary procedure comprises, for example, dipping activated carbon or carbon black in an aqueous chloroplatinic acid solution and wet-reducing the chloroplatinic acid by a chemical or dry-reducing it by a reducing gas. One of the recently proposed procedures comprises forming a platinum sulfite complex salt and oxidizing the complex salt, thereby depositing fine platinum particles on carriers (Japanese Laid-open patent application No. 88,478/76), and another procedure comprises depositing platinum particles of less than 50 A onto carbon carriers from an aqueous chloroplatinic acid solution by sodium dithionate and hydrogen peroxide (Japanese Laid-open patent application No. 92,588/74).
On the other hand, procedures for preparing a colloid dispersion of platinum particles by means of polyvinyl alcohol as a protective colloid were reported in J. Am. Chem. Soc., 63, 2745 (1941) and recently in Hyomen (Surface) 17, (4), 279-289 (1979).
However, the former procedures require complicated steps and have such disadvantages as poor deposition of platinum particles onto carriers, depending upon the species of carriers. In the latter procedures using the protective colloid, no disclosure has been made yet at all as to influences by the presence of carbon carriers.